Question

Martensite in a mild steel is obtained by

• A. Annealing
• B. Normalising
• C. Hardening
• D. Martensite can not be formed

Hint:

Hardenability is not the same as hardness. Hardenability is the *ability to form martensite* (and thus harden). Low carbon steel can't be hardened significantly by quenching because its hardenability is too low. High carbon steel has higher hardenability, and alloy steels have the highest hardenability.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Mild steel is a type of low-carbon steel, typically with a carbon content of less than 0.25%. The formation of martensite requires cooling the steel from the austenitic state at a rate faster than its Critical Cooling Rate (CCR). The CCR is the minimum cooling rate required to avoid the formation of diffusional products like ferrite and pearlite.

Step 2: Detailed Explanation:
The ability of a steel to form martensite is known as its hardenability. Hardenability is strongly influenced by the carbon content and alloying elements.

• Effect of Carbon: Low carbon content significantly decreases hardenability. For mild steel, the Time-Temperature-Transformation (TTT) curve is shifted very far to the left. The "nose" of the curve, which represents the start of the ferrite/pearlite transformation, occurs at extremely short times (fractions of a second).

• Critical Cooling Rate (CCR): Because the transformation to ferrite/pearlite starts so quickly, the CCR for plain mild steel is extremely high. It is so high that it is practically impossible to achieve, even with the most severe quenching mediums like iced brine. The heat cannot be extracted from the core of the workpiece fast enough to prevent the austenite from transforming into ferrite and pearlite.

• Other Processes: Annealing and Normalising involve slow cooling rates, which are designed to produce pearlite and ferrite, not martensite. Hardening (quenching) is the correct process in principle, but it is ineffective for mild steel due to its extremely low hardenability.


Step 3: Final Answer:
Due to its very low carbon content and consequently extremely low hardenability, it is practically impossible to quench mild steel fast enough to form a fully martensitic structure. Therefore, for all practical purposes, martensite cannot be formed in mild steel.